Problem: Which of these numbers is prime? ${33,\ 53,\ 55,\ 63,\ 92}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 33 are 1, 3, 11, and 33. The factors of 53 are 1 and 53. The factors of 55 are 1, 5, 11, and 55. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 92 are 1, 2, 4, 23, 46, and 92. Thus, 53 is a prime number.